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The science problem: timing techniques in high-energy astronomy

Tomaso Belloni visited the University of Southampton as a Leverhulme Visiting Professor between February and May 2012. Professor Belloni and I have worked together for a decade now, and our work is helping to define our understanding of the physical processes that take place around astrophysical black holes.

Accretion is the process whereby matter falls into the gravitational potential well of a black hole or other gravitating object, and as it does so converts potential to kinetic energy and ultimately to feedback to the surrounding environment in the form of radiation and kinetic energy. Accretion of matter onto black holes is a phenomenon that takes place on very different scales, from stellar-mass black holes stripping gas from a companion star to supermassive black holes consuming gas at the centre of active galaxies. This process leads to two major observable phenomena: the emission of strong X-ray radiation from the inner parts of the accretion flow and the emission of very fast jets at speeds close to that of light. These two phenomena must be considered together in order to understand these extreme celestial systems. Understanding how accretion and feedback proceeds as a function of the rate of infalling matter is directly relevant to our understanding of how supermassive black holes and galaxies coevolved since the big bang.

The project which Professor Belloni and I are working on is aimed at extending previous collaborative work on the connection between accretion and jet ejection. By reviewing existing information and analysing selected data sets, the goal is to design a unified model to describe the radio, infrared and X-ray emission from systems of very different sizes. The project is already bearing fruit in the form of a number of new analyses, and during the Leverhulme-funded visit this year we co-authored an invited review article for Science.

The puzzle of accretion in a close binary system. The companion star (on the right) loses mass to a black hole in which it is bound in a binary system. As the matter spirals towards the black hole it forms a disc which glows brightly in X-rays (on the left) but which also – somewhat mysteriously – produces jets of matter which flow away at greater than 90% of the speed of light.